The present invention relates to the surface treatment of metal bodies, and more particularly, to metal foils such as copper foils which are used in the production of a variety of products including printed circuit boards. More particularly, the invention relates to surface treatments for improving the properties of metallic bodies such as copper foils to maintain a bright copper tone during long storage and throughout lamination procedures conducted under heat and pressure while at the same time retaining the solderability and/or solder-wettability of the surface of the copper foil.
Printed circuit boards are currently used as the substrate materials in a wide variety of electronic devices. Typically, these boards are fabricated from a thin sheet of copper foil laminated to either a fiberglass/epoxy hard board or, in some instances, flexible plastic substrates. During the latter stages of the fabrication, the copper foil is printed with the necessary circuit pattern, and the unnecessary portions of the copper foil are then etched away to provide the desired interconnecting circuitry between various components in the electronic circuit design.
Copper foils used in such applications are prepared generally either by electrolytic deposition or a rolling technique. When the copper foil is produced electrolytically, the copper foil contains a matte or rough side and a shiny side. The side laminated to the plastic substrates generally is the matte side. Whether electrolytically formed copper foil or rolled copper foil is used, the surface of the foils thus formed are not readily amenable to the production of adequate bond strength after lamination. Therefore, the foil must be treated by additional chemical processes to improve its properties including bondability to resin surfaces, oxidation-resistance, corrosion-resistance, etc. The shiny side of the copper foils are treated to prevent oxidation during storage or lamination under heat and pressure. Various techniques have been suggested and utilized to improve the adhesion of the matte side of the copper foil to various polymeric substrates. One such practice for achieving adhesion between copper foil and insulating polymeric substrates has been to roughen the copper surface.
Surface roughening has been achieved by several means. The electrodeposited copper foils can be electroformed with a rough surface. On top of this rough surface further roughening is carried out by applying a high surface area treatment. These treatments may be a copper deposited electrolytically in nodular or powder form, or a copper oxide which grows nodular or dendritic, among others. Often times the rolled copper foil has mechanical roughness imparted to it during rolling or by subsequent abrasion. The rolled foils also are conventionally treated with surface area increasing nodular copper or copper oxide treatments.
These surface roughening treatments increase adhesion to the polymers by forming a mechanical interlock with the resin. The mechanical interlock is formed when an adhesive in its liquid state is applied and then cured or when the resin melts and flows prior to cure during lamination. The polymers flow around the roughened surface area treatments to form the mechanical interlock.
There are several factors contributing to the adhesion measured between the copper foil and the polymeric resin. Some of these are surface area, type of roughness, wettability, chemical bond formation, type of chemical bond, formation of interpenetrating networks, and properties of the adhering materials.
During an adhesion test the interlocked resin and copper often adhere well enough that failure occurs within the resin, a cohesive failure. With some resins the mechanical interlocking of treatment and resin does not result in the desired high adhesion and failure occurs at the interface between resin and copper, an adhesive failure.
The surface roughening that has been used to enhance adhesion between copper and polymeric resin substrates may cause difficulties in the manufacture of PCBs and contribute to poor PCB performance. In the subtractive copper etching process additional etching time is required to remove the dendrites or nodules embedded in the resin. This not only slows down the production process but contributes to greater line loss due to the lateral etching of the copper line's sidewalls. The surface roughening contributes to poor PCB electrical performance by degrading high frequency electrical signals. The necessity of having a rough base foil has limited other properties, such as tensile strength and elongation, that produce good laminate and PCB performance. The dendritic or nodular surface roughening treatments are difficult to apply, requiring special equipment in the case of electrolytic treatment, and special chemicals in the case of the oxide treatments.
The bonding strength of the foils to the polymeric substrates can also be improved by coating the foils with materials which are capable of enhancing the adhesion between the foil and the polymeric substrates. Various materials have been suggested in the literature as adhesion-promoting compounds, and these include organic materials such as phenol resins, epoxy resins, urethanes, silanes, polyvinyl butyral resins, etc. It also has been suggested to deposit layers of various metals and metal alloys to improve the adhesion between the copper foil and the polymeric substrates.
U.S. Pat. No. 3,585,010 (Luce et al) describes a conductive element for a printed circuit board comprising a copper foil and a metallic barrier layer which substantially reduces the staining of printed circuit boards. The metallic layer is a thin deposit of a metal selected from the group consisting of zinc, indium, nickel, cobalt, tin, brass and bronze. The barrier layer is applied to one side of the copper foil by standard electrodeposition procedures pertaining the particular metallic layer. The patentees also suggest that the metallic barrier layer does not have to be electrodeposited on the surface of the copper foil but may be applied by other means such as vapor deposition. After deposition of the barrier layer, the foil may be given additional treatments prior to lamination such as with a corrosion-inhibiting agent.
U.S. Pat. No. 4,268,541 (Ikeda et al) describes a process for producing a material having a vapor-deposited metal layer useful particularly in forming recording materials. The process described in this patent comprises vapor depositing a layer of metal, a layer of different metals in contact with each other, a layer of a metal alloy, a layer of a metal and a metal compound in contact with each other or a layer of a metal compound as the metallic layer on a support or substrate which may be a polymeric material, a composite of a polymeric material and paper, woven or non-woven cloth or paper. Subsequent to the formation of a layer by vapor deposition, a second layer of an organic material is applied over the metallic layer by vapor deposition using an evaporable organic material. The layer of organic compound over the metal layer serves as a buffering layer for the metallic layer and renders the metallic layer formed by vapor deposition more slippable.
U.S. Pat. No. 4,383,003 (Lifshin et al) and its divisional U.S. Pat. No. 4,455,181 describe copper-clad laminates useful in preparing high resolution printed circuit patterns. Laminates are made by vapor depositing a film of zinc on a vapor-deposited copper film which is on a silica-coated aluminum carrier sheet, vapor depositing a silica film on the resulting zinc-copper foil, bonding the resulting body to a substrate, and then stripping the silica-coated aluminum carrier sheet from the copper clad laminate. One of the laminated products described in the '003 patent comprises a thin copper sheet, an ultra-thin film of zinc vapor-deposited on said copper sheet and a film of silica or alumina vapor-deposited on said zinc film. Optionally, a coating of a silane coupling agent is deposited over the silica film.